Increased demands on the running qualities of gears in gear drives require more and more the precision finish machining of the gear teeth. In most cases the pre-machined workpieces are case-hardened, and then fine machined to the finished dimensions on all their functional surfaces. In this process the fine machining of the tooth flanks in particular is a complicated, resource consuming, and hence expensive operation. In the interests of economic manufacture, but also in order to avoid having to provide for unnecessarily large hardening depths and to balance and minimize the wear on the left and right flanks of the fine machining tool, it is therefore attempted to keep the material allowance for the fine machining as small as possible. For the fine machining of the teeth this means in practice that the depth of cut per flank for material removal is only a few hundredths to at most two tenths of a millimeter. If, as is generally the case, the left and right flanks are machined in the same operation, this demands a very accurate aligning of the precut teeth relative to the fine machining tool, so that the latter can be brought exactly into the centre of the tooth spaces to be machined, in order to achieve uniform material removal from the left and right flanks of the workpiece.
In the aligning process most frequently encountered in practice on gear finishing machines, a non-contact functioning measuring probe—the aligning probe—is located at a geometrically suitable point in the working area of the machine, near the outer diameter of the precut teeth of the workpiece set up on the work spindle, the said probe operating on an inductive, optical or magnetic principle. By rotating the work spindle, the angular positions of all the tooth flanks are measured, and an average value of the tooth space centre lines calculated. This is compared with the desired value stored in the machine control system, and brought to coincide with the latter by rotating the work spindle. As well as the aligning itself, the measuring action described simultaneously permits the gear blank to be tested with respect to the presence of the tooth spaces and the specified number of teeth.
Experience in industrial gear manufacture has led to the conclusion that alongside the number of teeth and the presence of pre-machined teeth prior to the machining operation intended, additional properties of the gear blank should be tested in order to guarantee the process reliability demanded. These include the radial run-out state of the pre-cut teeth, the tooth flank machining allowance, the helix angle, and any possible irregularities in the pre-machining operation across the whole workpiece facewidth on all the teeth. These properties cannot be detected with hitherto known aligning devices. Their inspection therefore requires either a previous measurement on a separate suitable measuring device in an additional operation, or additional measuring devices in the working area of the machine, which are difficult to accommodate due to the space limitations, and which increase costs.